It is explicitly emphasized herein that the invention relates exclusively to an optical device suitable for forming optical images of fingerprints, and that it does not relate to equipment situated downstream from said device for the purposes of scanning/recording the resulting image, detecting characteristic points of fingerprints, and providing information about the person concerned.
In particular, devices for detecting fingerprints can be used to authenticate a person so as to authorize that person to perform some determined operation: under such circumstances, the device for detecting fingerprints is often accompanied by some other apparatus that the authenticated person is to be authorized to use (a computer, an automated teller machine (ATM), . . . ). The space available for receiving the fingerprint detector device is then small, and in particular the optical imaging device incorporated in the fingerprint detection device must be made to be as compact as possible, and in particular with thickness (or height) that is as small as possible. By way of concrete example, at least certain specifications require the optical imaging device to present a thickness (or height) that does not exceed about 5 millimeters (mm).
Various embodiments of optical imaging devices for forming optical images of fingerprints are already known, including for example the device disclosed in document EP 0 308 162 which presents the dispositions set out in the introduction. Nevertheless, those known devices do not enable practical requirements to be satisfied completely.